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  <h2>python/面向对象高阶/16-实现文件上下文管理(__enter__和__exit__)</h2>



  <p class="post-date">2020-12-21</p>
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<li>我们知道在操作文件对象的时候可以这么写</li>
</ul>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">with open(&#39;a.txt&#39;) as f:</span><br><span class="line">    &#39;代码块&#39;</span><br></pre></td></tr></table></figure>

<ul>
<li>上述叫做上下文管理协议，即with语句，为了让一个对象兼容with语句，必须在这个对象的类中声明<strong>enter</strong>和<strong>exit</strong>方法</li>
</ul>
<h1 id="一、上下文管理协议"><a href="#一、上下文管理协议" class="headerlink" title="一、上下文管理协议"></a>一、上下文管理协议</h1><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line">class Open:</span><br><span class="line">    def __init__(self, name):</span><br><span class="line">        self.name &#x3D; name</span><br><span class="line"></span><br><span class="line">    def __enter__(self):</span><br><span class="line">        print(&#39;出现with语句，对象的__enter__被触发，有返回值则赋值给as声明的变量&#39;)</span><br><span class="line">        # return self</span><br><span class="line">    def __exit__(self, exc_type, exc_val, exc_tb):</span><br><span class="line">        print(&#39;with中代码块执行完毕时执行我啊&#39;)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">with Open(&#39;a.txt&#39;) as f:</span><br><span class="line">    print(&#39;&#x3D;&#x3D;&#x3D;&#x3D;&#x3D;&gt;执行代码块&#39;)</span><br><span class="line">    # print(f,f.name)</span><br></pre></td></tr></table></figure>



<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">出现with语句,对象的__enter__被触发,有返回值则赋值给as声明的变量</span><br><span class="line">&#x3D;&#x3D;&#x3D;&#x3D;&#x3D;&gt;执行代码块</span><br><span class="line">with中代码块执行完毕时执行我啊</span><br></pre></td></tr></table></figure>

<ul>
<li><strong>exit</strong>()中的三个参数分别代表异常类型，异常值和追溯信息,with语句中代码块出现异常，则with后的代码都无法执行</li>
</ul>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br></pre></td><td class="code"><pre><span class="line">class Open:</span><br><span class="line">    def __init__(self, name):</span><br><span class="line">        self.name &#x3D; name</span><br><span class="line"></span><br><span class="line">    def __enter__(self):</span><br><span class="line">        print(&#39;出现with语句，对象的__enter__被触发，有返回值则赋值给as声明的变量&#39;)</span><br><span class="line"></span><br><span class="line">    def __exit__(self, exc_type, exc_val, exc_tb):</span><br><span class="line">        print(&#39;with中代码块执行完毕时执行我啊&#39;)</span><br><span class="line">        print(exc_type)</span><br><span class="line">        print(exc_val)</span><br><span class="line">        print(exc_tb)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">try:</span><br><span class="line">    with Open(&#39;a.txt&#39;) as f:</span><br><span class="line">        print(&#39;&#x3D;&#x3D;&#x3D;&#x3D;&#x3D;&gt;执行代码块&#39;)</span><br><span class="line">        raise AttributeError(&#39;***着火啦，救火啊***&#39;)</span><br><span class="line">except Exception as e:</span><br><span class="line">    print(e)</span><br></pre></td></tr></table></figure>



<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line">出现with语句，对象的__enter__被触发，有返回值则赋值给as声明的变量</span><br><span class="line">&#x3D;&#x3D;&#x3D;&#x3D;&#x3D;&gt;执行代码块</span><br><span class="line">with中代码块执行完毕时执行我啊</span><br><span class="line">&lt;class &#39;AttributeError&#39;&gt;</span><br><span class="line">***着火啦，救火啊***</span><br><span class="line">&lt;traceback object at 0x1065f1f88&gt;</span><br><span class="line">***着火啦，救火啊***</span><br></pre></td></tr></table></figure>

<ul>
<li>如果__exit()返回值为True,那么异常会被清空，就好像啥都没发生一样，with后的语句正常执行</li>
</ul>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br></pre></td><td class="code"><pre><span class="line">class Open:</span><br><span class="line">    def __init__(self, name):</span><br><span class="line">        self.name &#x3D; name</span><br><span class="line"></span><br><span class="line">    def __enter__(self):</span><br><span class="line">        print(&#39;出现with语句，对象的__enter__被触发，有返回值则赋值给as声明的变量&#39;)</span><br><span class="line"></span><br><span class="line">    def __exit__(self, exc_type, exc_val, exc_tb):</span><br><span class="line">        print(&#39;with中代码块执行完毕时执行我啊&#39;)</span><br><span class="line">        print(exc_type)</span><br><span class="line">        print(exc_val)</span><br><span class="line">        print(exc_tb)</span><br><span class="line">        return True</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">with Open(&#39;a.txt&#39;) as f:</span><br><span class="line">    print(&#39;&#x3D;&#x3D;&#x3D;&#x3D;&#x3D;&gt;执行代码块&#39;)</span><br><span class="line">    raise AttributeError(&#39;***着火啦，救火啊***&#39;)</span><br><span class="line">print(&#39;0&#39; * 100)  #-------------------------------&gt;会执行</span><br></pre></td></tr></table></figure>



<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line">出现with语句，对象的__enter__被触发，有返回值则赋值给as声明的变量</span><br><span class="line">&#x3D;&#x3D;&#x3D;&#x3D;&#x3D;&gt;执行代码块</span><br><span class="line">with中代码块执行完毕时执行我啊</span><br><span class="line">&lt;class &#39;AttributeError&#39;&gt;</span><br><span class="line">***着火啦，救火啊***</span><br><span class="line">&lt;traceback object at 0x1062ab048&gt;</span><br><span class="line">0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000</span><br></pre></td></tr></table></figure>

<h1 id="二、模拟open"><a href="#二、模拟open" class="headerlink" title="二、模拟open"></a>二、模拟open</h1><p><img src="https://tva1.sinaimg.cn/large/0081Kckwgy1gm2qwgesmbj30ft0830tg.jpg" alt="113-enter,exit-open.jpg"></p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br></pre></td><td class="code"><pre><span class="line">class Open:</span><br><span class="line">    def __init__(self, filepath, mode&#x3D;&#39;r&#39;, encoding&#x3D;&#39;utf-8&#39;):</span><br><span class="line">        self.filepath &#x3D; filepath</span><br><span class="line">        self.mode &#x3D; mode</span><br><span class="line">        self.encoding &#x3D; encoding</span><br><span class="line"></span><br><span class="line">    def __enter__(self):</span><br><span class="line">        # print(&#39;enter&#39;)</span><br><span class="line">        self.f &#x3D; open(self.filepath, mode&#x3D;self.mode, encoding&#x3D;self.encoding)</span><br><span class="line">        return self.f</span><br><span class="line"></span><br><span class="line">    def __exit__(self, exc_type, exc_val, exc_tb):</span><br><span class="line">        # print(&#39;exit&#39;)</span><br><span class="line">        self.f.close()</span><br><span class="line">        return True</span><br><span class="line"></span><br><span class="line">    def __getattr__(self, item):</span><br><span class="line">        return getattr(self.f, item)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">with Open(&#39;a.txt&#39;, &#39;w&#39;) as f:</span><br><span class="line">    print(f)</span><br><span class="line">    f.write(&#39;aaaaaa&#39;)</span><br><span class="line">    f.wasdf  #抛出异常，交给__exit__处理</span><br></pre></td></tr></table></figure>



<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">&lt;_io.TextIOWrapper name&#x3D;&#39;a.txt&#39; mode&#x3D;&#39;w&#39; encoding&#x3D;&#39;utf-8&#39;&gt;</span><br></pre></td></tr></table></figure>

<h1 id="三、优点"><a href="#三、优点" class="headerlink" title="三、优点"></a>三、优点</h1><ol>
<li>使用with语句的目的就是把代码块放入with中执行，with结束后，自动完成清理工作，无须手动干预</li>
<li>在需要管理一些资源比如文件，网络连接和锁的编程环境中，可以在<strong>exit</strong>中定制自动释放资源的机制，你无须再去关系这个问题，这将大有用处</li>
</ol>
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